(1) Field of the Invention
The present invention relates to a solid-state image sensor including micro-lenses for a plurality of light-receiving elements formed on a semiconductor substrate, and a manufacturing method thereof.
(2) Description of the Related Art
In recent years, miniaturization of cameras is progressing along with increase in pixel count of solid-state image sensors.
With digital still cameras and camera-equipped mobile phones, a shortening of an exit pupil distance is progressing following the miniaturization of cameras. At this point, exit pupil refers to a virtual image of a lens (or stop) as seen from a light-receiving area, and exit pupil distance refers to a distance between the light-receiving area and a lens (refer to FIG. 18).
FIG. 18 illustrates a cross-section diagram of a camera portion of a mobile phone. A lens 110 is installed on a frame 111 of the mobile phone, and a CCD image sensor 112 is provided in an interior of the mobile phone. A distance between the lens 110 and the CCD image sensor 112 is an exit pupil distance D. Although incidence of light around a center of the light-receiving area is perpendicular, a periphery of the light-receiving area is limited to incidence of, not perpendicular, but oblique light.
FIG. 1 is a cross-section diagram illustrating a physical relationship of light-receiving elements and micro-lenses in an existing solid-state image sensor for facilitating shortening of an exit pupil distance. Light rays from a light source in the diagram represent incident light from a lens. The left side of the diagram represents a cross-section of a central portion of valid pixels making up a light-receiving area, and the right side represents a cross section of a peripheral portion of valid pixels making up the light-receiving area. As shown on the left side of the diagram, an in-layer lens 3, a color filter 5, and a micro-lens 7 are formed directly above a light-receiving element 1, in a central portion of the light-receiving area. Whereas in a peripheral portion of the light-receiving area, the in-layer lens 3, the color filter 5, and the micro-lens 7, are formed above the light-receiving element 1, offset towards a center of the light-receiving area, as shown in the right side of the diagram. In this manner, an existing solid-state image sensor forms a micro-lens in an offset position above a light-receiving element in a CCD image sensor light-receiving area, as a countermeasure for shortening of exit pupil distance. With this, an improvement of a light-collection rate for incident oblique light in the peripheral portion of the light-receiving area is promoted.
Furthermore, in a solid-state image sensor disclosed in official publication of Japanese Laid-Open Patent Application No. 06-326284, a transparent film with a lower refractive index than micro-lens material is provided, and deterioration of sensitivity upon lens stop release is minimized.
However, according to existing technology mentioned above, the shortening of exit pupil distance in response to slimming of mobile phone cameras and digital still cameras is reaching its threshold. For example, if the shortening of exit pupil distance should progress further with regard to the configuration in FIG. 1, the light-receiving elements will no longer be able to perform light-collection as intended, as shown in FIG. 2, and a problem of shading arises due to lack of sensitivity in the peripheral portion of the light-receiving area. In other words, from a center of an image, sensitivity deteriorates as a periphery of the image is neared, and a deterioration of image quality arises in which darkening worsens towards the periphery of the image.
In addition, due to the shortening of exit pupil distance, it is necessary to perform mask alignment with greater precision during a micro-lens formation process to form micro-lenses that are offset in appropriate positions for a case in FIG. 2 as well, and position alignment is becoming difficult in terms of design and production.
Furthermore, although deterioration of sensitivity upon lens stop release is reduced in the solid-state image sensor disclosed in official publication of Japanese Laid-Open Patent Application No. 06-326284, the same problem exists with regard to the shortening of exit pupil distance.